This invention relates to the detection of substances and measurement of the quantities present. More particularly, it relates to qualitative and quantitative analysis utilizing Raman spectra of the substances under analysis.
The Raman effect, or Raman scattering, is well known. Briefly and simply, when a beam of light impinges on substances, light is scattered. This scattering is of several different types, the predominant type being Rayleigh scattering, wherein the wave length of the scattered light is the same as that of the incident light. In the type utilized in the present invention, Raman scattering, the scattered light is of different wave lengths than the incident light; photons are absorbed by the substance and re-emitted at higher and lower wave lengths. A Raman spectrum of a substance is constituted of Raman scattered light and is spread across a wave length band even if the incident light is monochromatic, that is, all of a single wave length. There is a separate Raman spectrum of a particular substance for, or associated with, each incident wave length. In practice, a monochromatic beam of incident light is always used in Raman spectroscopy because of the difficulties in obtaining spectral separation. When Raman and Rayleigh scattered light is resolved into a spectrum by a spectrograph, Raman lines will appear on both sides of the Rayleigh line. The Raman line or lines on the low frequency side (or low wave number side or high wave length side) of the Rayleigh line are more intense than those on the high frequency side and are called the Stokes line or lines; those on the high frequency side are called the anti-Stokes line or lines. Not all substances are Raman active; there must be a change in polarizability during molecular vibration in order that a substance be Raman active. Substances which do exhibit Raman spectra can be characterized by means of their spectra. Qualitative analysis of a substance can be accomplished by comparison of the locations of its Raman lines with those of known standards. Quantitative analysis can be accomplished by comparison of intensities of Raman lines; this is generally a linear relationship. Of course, spectra which are compared must result from exciting radiation of the same wave length. For purposes of this document, a substance is defined as any composition of matter, including a single element and a mixture or solution of chemical compounds.
Raman spectroscopy has numerous applications and is a major research tool. It is now a rapidly developing area, having been neglected for many years in favor of infrared spectroscopy and ultraviolet spectroscopy. Advances in the equipment available for Raman spectroscopy, particularly the development of lasers as a source of monochromatic light, have provided much impetus. A review of the field of Raman spectroscopy, including theory, applications, potential, and citations to additional literature is provided by two recent publications: Raman Spectroscopy, Long, McGraw-Hill, 1977 and Chemical Applications of Raman Spectroscopy, Grasselli et al., Wiley and Sons, 1981.
Though Raman spectroscopy is an important research technique and is used for qualitative and quantitative analysis, there has not been available a Raman analyzer, that is, apparatus which provides, rather than a spectrum, an output comprising indication of substances present in a sample and, in the case of a quantitative analyzer, numbers denoting the amounts present of the constituent substances of a sample. There has not been available a routine method of analysis utilizing the Raman effect which provides qualitative or quantitative results which need no further processing or interpretation. Further lacking has been universal Raman effect apparatus and methods; that is, those that can be used for a wide variety of samples without significant change to the apparatus being required when different substances are to be analyzed. There are significant advantages in effecting qualitative and/or quantitative analysis using Raman spectroscopy in place of or in addition to conventional analysis methods.